The present invention relates to a laser machining apparatus, and more particularly to a laser machining apparatus and its control method suitable for drilling holes in a circuit board formed by laminating an insulating layer and a conductive layer.
Generally, circuit boards include the type called multilayer circuit board formed by laminating an insulating layer and a conductive layer alternately, and such multilayer circuit board is effective for enhancing the mounting density, and has come to be used widely.
Specifically, in the multilayer circuit board, holes are drilled in the insulating layer, and the holes are filled with solder or conductive paste to achieve conduction between adjacent conductive layers.
As the processing technology for drilling holes in such insulating layer, laser machining is widely employed.
The wavelength of the laser beam to be used is selected at a wavelength easy to be absorbed in the insulating layer and easy to be reflected in the conductive layer. For example, if the insulating layer is a glass epoxy resin and the conductive layer is a copper foil, carbon dioxide laser beam is used, so that the insulating layer only can be removed selectively.
It is important to drill holes so as to achieve conduction securely between adjacent conductive layers.
A prior art is disclosed, for example, in Japanese Laid-open patent No. 2-92482. According to this publication, when drilling holes in the workpiece by irradiating laser beam, the direct outputted beam from the laser output unit and the reflected beam from the workpiece are detected by individual sensors, and the reflected beam quantity ratio is calculated from the both beam quantities, then the laser beam is controlled by comparison with the reference value.
More specifically, when the reflected beam quantity ratio becomes larger than a predetermined reference value, the output unit is stopped.
By such operation, damage of conductive layer by laser beam is prevented, and the processing time is shortened at the same time.
The prior art, however, had the following problems.
That is, in the conventional machining method, only the reflected beam from the workpiece was detected. If the surface of the conductive layer for reflecting the laser beam of the workpiece is contaminated to absorb the laser beam due to some cause, the level of the reflected beam may not reach the specified level. As a result, excessive machining or abnormal heating may occur. When machining the workpiece, therefore, holes may not be drilled to a desired shape. This is one of the problems of the prior art.
Moreover, when the laser beam passes a focusing lens for machining, in the case of machining the center portion of the processing area by the scanning mirror, the laser beam is irradiated vertically to the object of machining. In the case of machining of peripheral portion, however, the laser beam is not irradiated vertically to the object of machining, and the laser beam reflected from the workpiece may be dislocated from the intermediate optical system parts, or may be irradiated to the optical parts holding portion, and the laser beam reflected from the workpiece may be diffused on the way, and part of the reflected beam may not reach the reflected beam detector. As a result, detection of reflected beam is imperfect, thereby lowering the precision of judgement of machining state of drilled holes.
Or, if the table on which the workpiece is mounted is inclined, similarly, part of the laser beam reflected from the workpiece may not return to the detector of laser beam. As a result, the reflected beam is not detected accurately, and the precision of judgement of machining sate of drilled holes is lowered. The prior art involved also such problems.
In the light of the above background, it is a first object of the invention to set the condition of every one pulse output of laser capable of machining the workpiece securely beforehand, and to set the maximum number of laser pulse capable of machining the workpiece securely beforehand. This is intended to control so as to stop the laser output unit when reaching a specified level by comparison with the reference value before reaching the preset number of outputs. As a result, laser machining is advanced in speed. In addition, setting of the maximum number of outputs can prevent excessive machining. Therefore, the machining yield is high and drilling of high quality is realized.
To achieve the object:
A first step is for setting the number of laser outputs as the laser output condition for drilling the workpiece by plural times of laser pulse outputs. Thereafter, laser machining is executed.
A second step is for detecting the machined state of the workpiece during execution of laser machining, and judging if reaching a desired machining state or not by laser machining.
A third step is for finishing the laser machining when judging that the workpiece has reached the desired machining state if not reaching the set number of laser outputs. Otherwise laser machining is continued to the set number of laser pulse.
Thus, the laser machining is carried out in three steps.
It is a second object to detect the laser light reflected from the workpiece by the detector, and correct the intensity signal of the detected laser light. That is, the laser light reflected from the workpiece is detected by the detector, the intensity signal of the detected laser light is corrected, and the intensity of the laser light reflected from the workpiece is calculated accurately. Thus, the machining state of drilling is detected accurately, and drilling of high quality is realized.
To achieve the object, the invention provides a control method of laser machining apparatus comprising the steps of:
detecting the reflected light intensity of the workpiece by a reflected light detector,
calculating the reflected light intensity right after reflection from the workpiece on the basis of the data of the reflected light distribution table storing the rate of change of the reflected light from the workpiece reaching the reflected light detector in every machining position of the workpiece, and
controlling the laser output unit on the basis of the comparison between the calculated reflected light intensity and the reference value.